Up to now, efforts have been made to develop recording systems which can repeatedly record and erase an image on and from a thermoreversible recording medium which becomes black, or transparent and colorless depending upon quantities of thermal energy applied thereto.
Japanese patent laid-open publications No. Sho 57-77140 and Hei 2-188294 propose examples of thermographic materials for such a recording medium.
The former publication exemplifies a thermoreversible recording medium comprising layers of thermoreversible material of a whitening group applied on the surface of a glass or plastic substrate. This material inverts its state at two transition temperatures t.sub.1 and t.sub.2 (t.sub.1 &lt;t.sub.2). When heated above the temperature t.sub.2 for a given period of time, the material becomes white. On the other hand, when heated above t.sub.1 but below t.sub.2 for a second given period of time, the material becomes transparent and colorless. Therefore, heating elements of a thermal head associated with an image to be recorded are heated above t.sub.2, while heating elements associated with an image to be erased are heated above t.sub.1 but under t.sub.2.
The latter publication discloses a thermoreversible medium including a thermoreversible material of a dye group. When the recording medium contains a dye whose transparency or color changes with temperatures, the medium can be repeatedly used for recording and erasing images such as letters and symbols thereon and therefrom, respectively, similarly to the foregoing thermoreversible medium of the whitening group.
The principle of the recording system will be described hereinafter. When a first energy (h.sub.1) is applied from a dynamic heat source such as a thermal head, the thermoreversible material is developed to form a first dark image (in black). The image is maintained as it is in a normal environment (temperature and humidity), but is erasable when a second energy (h.sub.2) is applied thereto. When the first energy (h.sub.1) is applied again, a second image can be formed. Thus, the recording and erasing can be performed repeatedly.
FIG. 1 of the accompanying drawings is a schematic view showing the configuration of the foregoing recording medium 1, which comprises a protective film 14, a recording layer 15 including materials such as a dye, an agent for making an image visible/invisible and a binder, and a substrate 16. When the first large energy (h.sub.1) of 200.degree. to 300.degree. C. is applied onto the recording medium 1 for a short period of time, e.g. 1 to 3 ms, in the direction shown by an arrow A, a black image is formed on the recording medium 1, for example. Conversely, when the second small energy (h.sub.2) of 80.degree.-160.degree. C. is applied to the recording medium 1 for a relatively long period of time, e.g. 5 ms to 2 sec, in the direction of the arrow A, the Image is erased from the recording medium.
Specifically, the recording layer 15 includes an agent for making the image visible/invisible which becomes acid and salt in response to an applied energy, and a leuco dye whose color changes with variations of acidity. FIG. 2 shows phenyl carbonate and organic amine salt as an example of the agent for making the image visible/invisible. FIG. 3 (a) shows a colorless leuco compound and FIG. 3 (b) shows a colored leuco compound.
The agent for making the image visible/invisible becomes acid when it is heated above the temperature t.sub.2, so that lactone rings of the leuco dye are opened. Thus, the leuco dye becomes colored. When heated above the temperature t.sub.1 but under the temperature t.sub.2, the agent for making the image visible/invisible changes to alkaline, so that the opened lactone rings are closed. Therefore, the leuco dye becomes colorless.
This recording medium has characteristics as shown in FIGS. 4 and 5. In FIG. 4, the abscissa represents a period of time for voltage supply, and the ordinate represents a recording density. From FIG. 4, it can be seen that the recording medium has the maximum recording density of 1.2 when the recording medium is applied with a voltage for approximately 3 ms. In FIG. 5, the abscissa denotes an erasing temperature and the ordinate a recording density after erasure. In this case, the recording medium is applied with the voltage for 3 ms (i.e. the state where the recording medium has a recording density of 1.2) and is then heated by a heat roller, a thermal head or the like. FIG. 5 shows that the recording medium is completely free from an image near 120.degree. C. to 150.degree. C. (i.e. the state where the recording medium is similar to that having the density 0.15 prior to the recording).
The erasing characteristics are also shown in FIGS. 6 and 7, which are obtained in a different manner. FIG. 6 shows a completely black pattern 41 formed by the thermal head on the recording medium 1. FIG. 7 shows the erasing characteristic of the recording system which erases the black pattern of FIG. 6. An energy of 1.0 mJ/dot and an energy of 0.6 mj/dot are applied to the recording medium in the direction shown by an arrow B for the recording and erasing, respectively. Referring to FIG. 7, it can be seen that the erasing is not complete at the beginning of the erasing process (i.e. about the first to 30th lines in the black image) and substantially after the 300th and succeeding lines of the black image.
The head portion of the recorded image is not erased because the thermal head does not reach its effective temperature. This is because heating elements of the thermal head take a certain period of time to become effective even when thermal head is left at room temperature (without applying a voltage thereto for a while) and is heated under such a condition. The thermal head is not elevated to its effective temperature until the tenth line is being erased. In other words, the thermal head is unstable in its operation until it is sufficiently activated.
The reason why the image is not erased in a portion following a 300th line is that the heating elements become too hot in the heated thermal head. Two kinds of energy are reserved in the thermal head. One is a part of the energy generated by the heating elements and the other is the energy which is used to erase a previous line and both energies remain accumulated around the heating elements. Both of these energies raise the temperature of the heating elements which are repeatedly heated for every line. Thus, the thermal head becomes too hot to erase the recorded image.
FIG. 8 shows a comparison of erasing characteristics on a large recording medium of A4 size and a small recording medium of a card size. In FIG. 8, the ordinate represents the numerical order of a line to be erased, and the abscissa represents an erasing temperature. The larger the recording medium, the more incomplete the erasure.
The conventional recording and erasing system for the thermoreversible recording medium adopts a method in which energies are applied to the recorded image so as to make it invisible. In other words, the recorded image to be erased is heated at the temperature which is above t.sub.1 but under t.sub.2 as mentioned above.
As described so far, the thermoreversible recording medium tends to vary its reflectance and recording density somewhat depending upon its recording and erasing history. In other words, the recording medium shows different degress of reflectance and recording densities at the recorded and erased areas and at the areas which have never been recorded and erased. Therefore, incompletely erased images sometimes remain vaguely on the recording medium in a manner such that they are faintly visible. Prior art recording and erasing systems suffer from the problem that erasure is somewhat incomplete.
Furthermore, there are few recording mediums which are completely thermoreversible. Usually, the more often they are used, the poorer they become, and finally they will become unusable. During repeated use, the recording medium extensively undergoes physical and chemical changes so that it may become worn out. Furthermore, the recording medium may have its protective film and thermoreversible film damaged by heat and pressure applied thereto via the thermal head as a heating means. Therefore, the user has to determine whether or not the recording medium in use is still usable, and remove the unusable recording medium. If such a unusable recording medium is continuously used since the user is not aware of its reduced performance, either recording or erasing cannot be carried out thereon, which will be inconvenient to the user.
Such determination on the performance of the recording medium will be troublesome to the user. Sometimes, the user might throw away a still usable recording medium, or recording might be performed to no avail on an unusable recording medium.